Balloon catheters are being increasingly used to reach remote locations in the body of a patient. When the target is a soft tissue site, the vascular system in the region often consists of vessels of very small diameter. The vessels are also often convoluted, making many sharp twist and bends. To navigate these small tortuous vessels requires a catheter having a correspondingly small outside diameter. The predominant method for achieving small diameters is to use catheters having very thin walls. However, as the walls of a catheter get thinner, they tend to lose their torsional and longitudinal rigidity. Sufficient torsional rigidity must be maintained to permit steering of the catheter through the vessel and sufficient longitudinal rigidity must remain to allow the catheter to be advanced (i.e., pushed) through the vessel. Furthermore, thin wall tubes have a tendency to crimp or kink when bent in a small radius. This can result in the binding of guide wires within the catheter in the vessel which normally depends on prior advancement of a guide wire.
The problem of achieving a small tube diameter while still having sufficient torsional control and longitudinal control and kink resistance is compounded in cases where a catheter having more than one channel or tube is required, such as in the treatment of atherosclerotic lesions in the arteries of the brain, in which a balloon catheter is used that is similar to, but much smaller than, that employed for percutaneous transluminal coronary angioplasty. Such a catheter is typically composed of two tubes, an outer tube that, at or near its distal end, is in fluid communication with a balloon-like structure and an inner tube through which a guide wire or other instrumentation may be passed. The annular space between the two tubes provides a channel through which liquids can be introduced and removed to inflate and deflate the balloon.
The general approach to accommodating the need for small outside diameter catheters is to reduce the size of guide wires and the wall thickness of both tubes making up a balloon catheter. However, there are limits to the extent to which these dimensional reductions can be taken. If the diameter of the guide wire is reduced too much, the guide wire will lose its ability to effectively transmit torsional and axial (i.e., longitudinal) forces necessary to steer and advance the guide wire through tortuous vascular systems. Thus, if the diameter of the wire is to be maintained at a functional dimension, then the first impulse is to reduce overall catheter size by reducing the wall thickness of the tubular portions of the catheters.
Unfortunately, this can result in loss of cross-sectional circularity of either or both the inside and outside tubes, resulting in crimping or kinking. If the inner tube kinks, then the guide wire will become bound within the tube's lumen and can no longer be advanced through the vascular system. If the outer tube kinks, it may cause the inner tube to close down and bind the guide wire or it may constrict, even close down, the annular space between the tubes making it difficult or impossible to expand and deflate the balloon structure.
Thus, there is a need for a balloon catheter structure combining a thin overall cross-section with controlled flexibility, kink resistance and the structural strength to withstand the high pressures created during the inflation of the balloon portion of the catheter.